The following reasons could be why NB IoT Digital Key unlock is not working
1. You don't have a 3G/4G/Wi-Fi connection. Turn on 3G/4G/Wi-Fi on your phone. To unlock the door, the unlock command goes from your phone app up into the cloud (in the normal way anything goes up or down to the cloud and that is via 3G/4G/Wi-Fi), where it is converted to a NB IoT signal/checks the time/digital key ID to see if you are authorised, before coming down to the lock to unlock it. Don't worry about using much data on your mobile plans, each unlock command is tiny - only around 8bytes. Also the commands are encrypted too.
2. The NB IoT signal is weak or intermittent your distributor/telco will ensure that your NB IoT signal strength is strong on your site before a lock is installed (locks are designed not to be moved around after being installed), but on a rare occasion, the NB IoT network might be down, or intermittent. The app commentary will tell you if the NB IoT signal is poor or intermittent, and so you should try again, and if the problem repeats please call your local Telco's 24 hour support line. If the problem persists gain access to your property room by using the metal key override or the keycard/token (keycard/tokens do not unlock over NB IoT network).
3. The battery level is too low. Gain access to your property through other means (metal key or keycard/token and change to new batteries.